The process of inflammation is mediated by both circulating and resident monocytes and the cells with which they interact, e.g. vascular endothelium and smooth muscle cells. The appearance of inflammatory cells at the site of tissue injury is a characteristic of almost any immune-related disease. Infiltrating monocytes mediate the initiation and progression of damage by direct cytotoxicity, the secretion of soluble factors, or by regulating the immune response. These include the expression of adhesion molecules on monocytes and underlying vascular endothelium and the release of cytokines, chemokines, tissue-destructive metalloproteases and reactive oxygen species.
During the early, specific phase of the host defense response, production of interferon-gamma (IFN-γ) by natural killer cells plays an important role in setting off acute inflammation, primarily because of the activating effects of IFN-γ on the adhesive properties of endothelial cells and on mediator production by mononuclear phagocytes. High-level production of IFN-γ during this phase of host defense is a hallmark of a T-helper 1 (TH1)-type reaction, characterized by activation of the antimicrobial activity of macrophages and by triggering a cascade of inflammatory reactions.
Macrophages play a significant role in the host defense mechanism. These cells reside in various tissues and are among the first cells of any organ to be exposed to infectious agents and to become activated in response to an insult. Upon activation macrophages participate actively in the onset of inflammation by releasing cytokines that amplify the initial inflammatory response. These inflammatory cytokines include: bioactive lipids (prostaglandins and leukotrienes), reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) that exert cytotoxic effects against pathogens and tumor cells. Activation of the host immune system by Gram-negative bacteria can be reproduced in vitro by incubation of cells with lipopolysaccharide (LPS, a predominant glycolipid in the outer membrane of Gram-negative bacteria) and IFN-γ acting as a pro-inflammatory cytokine. LPS and IFN-γ stimulate macrophage cells to an activated state, priming them for antimicrobial activity, increased killing of intracellular pathogens, and antigen processing and presentation to lymphocytes. These biological effects are mediated by up-regulating MHC class II expression and increasing release of nitric oxide (NO) and pro-inflammatory cytokines.
Several early signaling pathways have been identified in response to LPS-induced activation of macrophages, including mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase, and p38/stress-activated protein kinase as well as various members of the Src family of protein tyrosine kinases and Vav. LPS, commonly known as a second messenger, synergizes with IFN-γ in the stimulation of macrophages by activating NF-κB which results in the transcriptional up-regulation of a number of genes involved in the cell-mediated immune response. As a result of this activation sequence, macrophages express enzymes involved in inflammation, such as inducible NO-synthase (iNOS, the enzyme responsible for NO synthesis), cyclooxygenase-2 (COX-2, the enzyme responsible for the high output synthesis of PGs), and matrix metalloproteinases.
Nitric oxide is a highly reactive nitrogen radical implicated in multiple biologic processes ranging from endothelium-dependent relaxation to long-term potentiation in neuronal synapses and macrophage tumoricidal activity. Its formation is regulated by a family of enzymes, known as nitric oxide synthase (NOS). At least three distinct, but functionally and structurally related, isoforms of NOS have been identified in mammalian cells.
The macrophage iNOS regulates NO synthesis over a period of several hours following stimulation with LPS. Changes in NO production in iNOS-expressing cells usually correlate with similar changes in iNOS mRNA levels, indicating that a major part of iNOS regulation occurs at the level of transcription. The promoter region of the iNOS gene contains several consensus sequences for the binding of transcriptional factors, such as NF-κB, activator protein-1 (AP-1), and various other proteins. Of the latter proteins, members of the NF-κB family appear to be essential components for enhanced iNOS gene expression in macrophages exposed to the active components of the LPS endotoxin. In unstimulated cells, NF-κB is retained in the cytoplasm by binding to IκB. The NF-κB-IκB complex is released in response to activated signaling cascades, upon which NF-κB translocates to the nucleus, and activates responsive gene elements.
As a mediator of macrophage cytotoxicity, NO is proven to be beneficial when released in physiological amounts. However, despite its beneficial role in host defense mechanisms, excessive NO production by activated macrophages has been implicated in the pathogenesis of several acute and chronic inflammatory diseases including arthritis, inflammation of the upper respiratory tract, lung inflammation, and the enhancement of human immunodeficiency virus (HIV) replication in primary human macrophages. Also, iNOS-dependent tissue destruction has been seen in several rodent autoimmunity models, such as experimental allergic encephalitis (EAE) and uveitis (EAU), inflammatory arthritis, and immune complex glomerulonephritis. Furthermore, it has been demonstrated that iNOS promotes tumor angiogenesis and metastasis.
Therefore, the regulation of NO production by activated macrophages, and consequently inflammation, is critical to limiting damage to host tissues without compromising the immune response elicited by the host to combat infectious agents.
In view of the importance of monocyte activation in immune responses, the further characterization and manipulation of the pathways regulating activation are of great interest.
Relevant Literature
The role of nitric oxide in immune function is discussed in MacMicking et al. (1997) Annu. Rev. Immunol. 15:323; Albina and Reichner (1998) Cancer Metastasis Rev. 17:39; Xie et al. (1994) J. Biol. Chem. 269:4705; and Nathan (1995) Cell 82:873.